The present invention relates to a method and system for automatically correcting image registration in an image transfer system comprising a plurality of moving, image-forming media on each of which an image is formed in response to a corresponding start-of-page (SOP) signal. An image carrier is moved past each of the image-forming media and is brought into contact therewith in a respective transfer zone.
The problem of correcting image registration occurs, for example, in a color copier or printer, in which it is essential for obtaining a good image quality that the various color separations are superimposed correctly on the image carrier. For example, a four color reproduction system comprises four image-forming media corresponding to the four basic colors, i.e. yellow, cyan, magenta and black.
The image-forming media may be drums or belts on which a developed toner image in the corresponding color can be formed by any known process, e.g. by a direst induction process or by a xerographic process. In the latter case, the surface of the image-forming medium is formed by a photoconductor on which a charge image is formed by image-wise exposure with light and then the charge image is developed with toner.
The image carrier may be a sheet of copying paper on which the desired image is to be recorded or an intermediate carrier (belt or drum) from which the color image is then transferred to the final recording medium in a second transfer step. In any case, the image carrier is successively moved through the various transfer zones, so that the developed single-color images (color separations) are superposed on the image carrier to form the desired multiple-color or full-color image.
In each transfer zone the image carrier is brought into contact with the corresponding image-forming medium in a nip which may be constituted by the image-forming medium and the image carrier themselves or, in case of a belt, by rollers supporting the belt. In order to obtain a correct registration of the superimposed images, the mechanical components of the transfer system have to be adjusted correctly, and the timings of the SOP signals, which define the positions of the leading edge of the image on the respective image-forming medium, have to be selected properly, such that the leading edges of all images will coincide on the image carrier. In the course of time, however, the mechanical components are subject to wear or aging, thermal expansion and the like, so that the image registration may be altered to an extent which is not acceptable in a high resolution system.
U.S. Pat. No. 4,937,664 discloses a laser printer in which the image registration can be checked and corrected automatically, for example in the warming-up phase each time the printer is switched on. To this end, the image-forming units are arranged to form registration marks on the image carrier. A detector for detecting these registration marks is arranged downstream of the image-forming units and compares the timings at which the registration marks are detected to corresponding target values. In case of a deviation, a mechanical component of the associated image-forming unit, e.g. the optical exposure system is readjusted by means of an actuator in order to compensate for the misregistration. The registration marks formed on the image carrier are then erased again, so that the system will not be confused when new marks are generated in a subsequent correction cycle.
In conventional color copiers or printers, in general, the drive systems for the various image-forming media and the image carrier are mechanically coupled to one another through gears or the like, so that all image-forming media are forcibly driven at the same speed as the image carrier. This facilitates the adjustment of image registration, but has the drawback that a rather complex mechanical system is required. With increasing resolution of the printer and, accordingly, increasing accuracy requirements, it becomes increasingly difficult and expensive to suppress effects resulting from gear play, manufacturing tolerances of the gear teeth and the like to an acceptable limit.
Theoretically, the speeds of all image-forming media should be exactly identical, because they are all held in contact with the same image carrier. However, it is found that in practice the natural speeds of the image-forming media, i.e. the speeds the image-forming media would acquire if they were allowed to idle, are slightly different from one another. These speed differences may for example result from variations in the thickness of the image carrier belt, variations in the thickness of the toner layer, and from slight elastic deformation of the image-forming medium or the image carrier due to forces acting in the nip in the transfer zone. When the image-forming media are forcibly driven at the same speed, these differences in the natural speeds may result in undesirably high tangential forces or torques which act upon the image carrier in the transfer zone and may impair the image quality or the lifetime of the image carrier and other mechanical components.
U.S. Pat. No. 4,705,385 discloses a color printer in which the image carrier and the image-forming medium are driven independently from one another with controllable speeds. There is only provided a single image-forming medium in the form of a photoconductive belt,the length of which is an integer multiple of the circumferential length of the image carrier. The various color separations are formed one after the other on the same photoconductive belt and are transferred to the image carrier after each complete revolution of the latter. The drive system for the photoconductor serves as a master to which the drive system of the image carrier is slaved. More specifically, servo control devices keep track of the displacements of the photoconductor and the image carrier, and when the image carrier has fallen behind or gotten ahead of the photoconductor belt, the displacement of the image carrier is corrected within a short time interval in which image free seam areas of the belts are in contact with each other. Thus, all color separations will be superimposed on the image carrier with correct image registration. Since this system employs only a single image-forming medium, there is no need to cope with registration errors resulting from speed differences between image-forming media. However, the use of only a single image-forming medium leads to losses in productivity.